Dissolution method and apparatus

ABSTRACT

A dissolution system employs a tapered vessel that has a wide, upper mouth and a narrow, lower neck. A liquid is introduced into the vessel through the lower neck and flows upwardly. A powder to be dissolved in the solution is introduced onto the liquid surface at the wide, upper mouth of the vessel and sinks downwardly into the liquid. Powder that does not quickly dissolve gravitates down towards the neck where dissolution is facilitated by the increased purity and the increased flow per unit area of the incoming liquid. Settling of the powder at the base of the vessel is prevented by the incoming fluid flow. The dissolved solution is withdrawn through an outlet port.

RELATED APPLICATION DATA

This application is a continuation-in-part of copending U.S. applicationSer. No. 07/051,054, filed May 15, 1987now U.S. Pat. No. 4,812,239,incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to apparatuses for continuously dissolvingpowders into liquid solutions. It is illustrated with reference to asystem for producing dialysate solution used in hemodialysis therapy buthas many other applications as well.

BACKGROUND AND SUMMARY OF THE INVENTION

Dialysate solution used in hemodialysis therapy is generally formed bymixing together controlled proportions of liquid dialysate concentrateand water using a "proportioning system." A first type of proportioningsystem, exemplified by U.S. Pat. Nos. 3,598,727 and 4,172,033 toWillock, 4,107,039 to Lindsay, Jr. et al. and 4,136,708 to Cosentino etal., operates by volumetrically metering liquid concentrate and water toa mixing point using positive displacement piston pumps. The preciseratio of liquid concentrate to water is generally determined by therelative volumes of the pump chambers and in some systems can be variedsomewhat by varying the relative travels of the pump pistons.

A second type of proportioning system, exemplified by U.S. Pat. Nos.4,202,760 to Storey et al., 4,293,409 to Riede et al., 4,508,622 toPolaschegg et al., 3,847,809 to Kopf and 4,082,667 to Seiler, Jr.,produces dialysate by introducing dialysate concentrate at a controlledrate into a flow of water. The conductivity of the resulting dialysateis detected downstream from the mix point and is used to regulate therate at which the liquid concentrate is introduced into the flow ofwater.

Dialysate proportioning systems are generally included as integral partsof hemodialysis therapy machines, serving to mix the dialysate solutionon-line as part of the machine's operation. Preparation of the liquidconcentrate used in such systems, however, is an off-line, batch processperformed either by the operator of the dialysis machine or by a thirdparty concentrate vendor In either case, the process is the same. A drychemical mix of selected salts and other components is dissolved in alarge tank of water. Dissolution of the dry chemical mix is effected bya mechanical stirring element or by a recirculation pump that withdrawswater from the tank and forcibly reintroduces it, thereby causingturbulence that promotes dissolution. Undissolved chemical that settlesto the bottom of the tank, however, tends to collect adjacent thesidewalls where it is not readily dissolved by recirculation or stirrertechniques. Exemplary of the apparatuses used to prepare concentrate isthe Renapak Concentrate Manufacturing System marketed by Renal Systems,Inc.

The current practice of preparing dialysate from liquid concentratesuffers from a number of drawbacks. First is the need for complexhydraulic equipment in those proportioning systems that rely onvolumetric metering of the components to be mixed. Such complexityincreases the cost and reduces the reliability of these systems. Secondis the cost, bulk and weight of the liquid concentrate itself. The costof the concentrate is substantial due to the cost of the concentratemanufacturing equipment and the labor costs associated with operatingthis equipment. The cost of concentrate purchased from third partyvendors is even higher due to the addition of the vendor's profit. Theshipping costs are substantial due to the concentrate's volume andweight. Handling costs at the user site are also substantial because theconcentrate generally cannot be handled by the regular nursing staff butinstead requires use of lift trucks or other such devices.

Accordingly, there remains a need for an improved method and apparatusfor preparing dialysate used in hemodialysis therapy.

More generally, there remains a need for an improved method andapparatus for dissolving powders into liquids.

The apparatus disclosed herein provides a method and apparatus forcreating dialysate solution directly from dry chemicals in a continuousprocess at the user site, thereby eliminating the intermediateconcentrate step and its attendant drawbacks.

The illustrated embodiment includes a drum for containing the drychemical mix, a conveyor belt, a dissolution vessel and monitoringequipment. The drum includes internal baffles that deposit the drychemical onto the end of the conveyor belt as the drum rotates. Theconveyor belt passes the chemical through a profile-determining gate andinto the dissolution vessel. The rate at which the belt delivers the drychemical to the dissolution vessel is governed by an electronic circuitthat measures the conductivity of the dialysate and adjusts the beltspeed as necessary to maintain the dialysate conductivity at a setvalue.

The preferred dissolution vessel used in the present invention isconical in shape, having a wider upper portion and a narrower lowerportion. Warm water is introduced in the lower portion of the vessel andflows upwardly to the water surface. As particles of the chemical dropfrom the conveyor onto the surface, some particles dissolveinstantaneously. Other particles, especially larger particles, sinkbelow the surface and gravitate downwardly through the flowing liquid.As a particle sinks to the lower portion of the vessel, it encountersprogressively less saline water at a progressively greater flow rate andturbulence. This environment promotes rapid dissolution of even cakedlumps of the chemical.

The dissolved solution (dialysate) flows out the dissolution vessel andinto a flow controller which regulates the rate at which dialysate issupplied to the downstream dialysate equipment. A restriction in theflow controller serves as a nucleation site for bubble formation. Adownstream deaeration pump causes the dialysate to be degassed

If it is necessary to adjust the pH of the dialysate solution, a smallpump, such as a peristaltic pump, a roller pump, or a cylinder typepump, can be used to inject an acidic fluid at a controlled rate intothe warm water provided to the dissolution vessel. Such a pump can bedriven from the same shaft that is used to drive the conveyor beltcarrying the dry chemical mix.

Desirably, two spaced apart pairs of electrodes are used to monitor thedialysate concentration and to regulate the conveyor speed (or othercontrol mechanism) appropriately. The first pair of electrodes measuresthe conductivity in the dissolution vessel itself and thus responds toshort term variations in the composition of the dialysate solution. Thesecond pair of conductivity probes measures the conductivity of thedialysate downstream from the dissolution vessel and is thus useful inmaintaining long term regulation of the dialysate solution composition.

These and other features and advantages of the present invention will bemore readily apparent with reference to the following detaileddescription, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a dissolution apparatus accordingto one embodiment of the present invention.

FIG. 2 is a perspective view of a portion of the apparatus of FIG. 1showing a dissolution vessel, a flow control vessel and part of a drypowder conveyor.

FIG. 3 is a sectional view taken along lines 3--3 of FIG. 2 showing thedissolution and flow control vessels.

FIG. 4 is a perspective view of another portion of the apparatus of FIG.1 showing the other end of the conveyor and a drum driving system.

FIG. 5 is a perspective view of a powder supply drum used in theapparatus of FIG. 1.

FIG. 6 is a section view taken along lines 6--6 of FIG. 4 showing amaterial conveyor assembly used in the apparatus of FIG. 1.

FIG. 7 is a sectional view taken along lines 7--7 of FIG. 6.

FIG. 8 is a sectional view taken along lines 8-8 of FIG. 6.

FIG. 9 is a schematic diagram of a control system used in the apparatusof FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

With reference to FIG. 1, a dissolution apparatus 20 according to thepresent invention includes a source 22 of dry powder, such as a drychemical dialysate mix, a source 24 of fluid and a dissolution vessel 26for dissolving the dry chemical into the fluid. The dry chemical in thisembodiment comprises a commercially prepared mixture of dialysis saltscomposed principally of sodium chloride and calcium chloride, togetherwith an assortment of other compounds, such as potassium chloride,magnesium chloride, acetate and dextrose. The fluid here is water at 38degrees Celsius. A transport system 28 is provided for transporting drychemical from dry powder source 22 to dissolution vessel 26 at acontrolled rate. A pump 30 is provided for introducing fluid from fluidsource 24 into dissolution vessel 26 through an inlet conduct 31. A pump32 can optionally be employed to add an acidic fluid from an acidicfluid source 34 into vessel 26 through inlet conduit 31 at a controlledrate when preparing bicarbonate dialysate.

Dissolution vessel 26 includes an outlet port 36 through which solutionflows from the dissolution vessel into an inlet port 82 of a flowcontroller vessel 38. Outlet port 36 and inlet port 82 cooperate to forma weir 37 between the dissolution and flow controller vessels which actsto regulate the fluid level in the dissolution vessel. Dissolutionapparatus 20 further includes at least one detector 40 for detecting acharacteristic of the solution and for producing an electrical outputsignal corresponding thereto. A second detector 42 can optionally beemployed to detect the same characteristic of the solution at a pointremote from detector 40. A control system 44 receives the outputsignal(s) from the detector(s) and varies the rate at which the drychemical or the fluid is introduced to the dissolution vessel 26 so asto substantially maintain the detected characteristic of the solution ata desired value.

DISSOLUTION VESSEL

Referring now to FIGS. 2 and 3, dissolution vessel 26 desirablycomprises a tapered vessel having a wider upper portion 53 for receivingdry chemical and a narrower lower portion 54 for receiving the flow ofincoming fluid. As particles of the dry chemical introduced fromtransport system 28 gravitate from the wider upper portion of the vesseldown to the narrower lower portion, they encounter progressively greaterfluid flow rates and progressively lesser fluid salinity. (At somepoint, theoretically, the force of gravity carrying each particledownward is precisely counteracted by the force of the water flowpushing the particle upward. Steady state equilibrium, however, is neverreached since the particle is continuously dissolving, reducing both itsmass and surface area.) Since undissolved particles are never allowed tosettle to the bottom, they continue to circulate in the turbulent fluiduntil they are completely dissolved. The flow rate and salinity profilesinherent in this tapered vessel configuration help contribute to thisrapid and complete dissolution of all chemical introduced into thevessel.

In some embodiments, powder dropped on the surface of the fluid invessel 26 may immediately flow out the weir 37 and into the flowcontroller vessel 38 without first sinking into the fluid anddissolving. Accordingly, the preferred embodiment is provided with apartially submerged ring 50 held in place near the top of vessel 26 byarms 52. Ring 50 blocks the surface flow of fluid through the weir 37and requires that any floating powder first gravitate downwardly intothe fluid before it can escape the dissolution vessel.

Disposed on the inner wall of vessel 26 are two electrodes 40a, 40b thatcomprise detector 40. The fluid characteristic normally detected bydetector 40 is the solution's concentration, detected by monitoring theconductivity of the fluid. When excited with an electrical signal, thecurrent through, and the voltage across electrodes 40a, 40b varies withfluid conductivity according to Ohm's law. These analog signals arecoupled to control system 44 (FIGS. 1, 9), which in turn regulates theaddition of fluid and/or chemical to vessel 26 so as to substantiallymaintain the conductivity of the solution at a desired level.

FLOW CONTROLLER

Dialysate solution produced in vessel 26 flows out withdrawal port 36and into an inlet port 82 of flow controller 38. Withdrawal port 36 andinlet port 82 are forced into proximity by a coupling frame 84 comprisedof a U-shaped rod 86, a plate 88 and wing nuts 90. A watertight sealbetween ports 36 and 82 is established by a compressed 0-ring 92disposed between the ports.

Flow controller 38 comprises a float controlled valve formed in thebottom of a molded vessel 98. A float member 104 floating in thedialysate solution in molded vessel 98 regulates the position of atapered occluding member 100 in an orifice 102 in the bottom of thevessel. A conduit carries the dialysate from orifice 102 to a dialysismachine (not shown). In addition to being a component of the floatcontrolled valve, occluded orifice 102 also serves as a nucleation sitefor bubble formation. Bubbles formed at this site can be removed by adownstream bubble trap or deaeration pump. An extension 106 of occludingmember 100 extends vertically and serves as a handle by which a user canremove float 104 from vessel 98.

Flow controller 38 dynamically regulates the flow of dialysate solutionto the downstream dialysis equipment so as to maintain a constant fluidvolume in dissolution vessel 26. If, for example, the dialysis equipmenttries to draw more fluid from flow controller 38 than dissolution vessel26 is able to supply, the fluid level in the flow controller vesseldrops. When the fluid level in the flow controller vessel drops, float104 also drops. When float 104 drops, occluding member 100 drops furtherinto orifice 102, thereby reducing fluid flow out of the flowcontroller. This reduced fluid flow out of the flow controller causesthe fluid level within the flow controller vessel to rise untilequilibrium is again established with the fluid level in dissolutionvessel 26.

DRUM ASSEMBLY

Referring now to FIGS. 4 and 5, the dry chemical source used in thepreferred embodiment of the present invention comprises a rotatable drum22. Drum 22 has first and second ends 120, 122 and a cylindricalsidewall 124 extending therebetween. The drum is mounted for horizontalaxial rotation on rollers 128, 130 which extend from a panel 132 and onwhich the drum rests. Roller 128 is driven by a motor (not shown)positioned behind panel 132. Protruding through both panel end 132 andfirst end -20 of drum 22 is a hub 126 about which the drum rotates. Hub126 is a component of conveyor 28 and is bolted to conveyor support 133(FIGS. 2, 6), which is in turn attached to panel 132.

Inside drum 22 is at least one vertically inclined blade 140 (FIG. 5)extending along sidewall 124. Each blade 140 lifts dry chemical from thebottom of the drum and causes it to slide towards the first end 120 ofthe drum as the drum rotates. Each end of each blade is held in place bya rod 142 which extends through the drum and is held in place by a wingnut 144 at second end 122.

Drum 22 further includes at least one radially extending chute member146 located near first end 120 of the drum. Each chute member 146 liftsdry chemical from the bottom of the drum and directs it onto centrallylocated conveyor 28 as the drum rotates. Each chute member 146 is fixedin place at its outer end by one of rods 142 and is fixed in place atits inner end by a rod 148.

In addition to their principal functions of transferring dry chemicalonto conveyor 28, blades 140 and chutes 146 also serve to agitate andmix the chemical. This can be used to advantage if it is desired tomodify a standard, commercially available dry chemical mix with certainadditional components. The additional components are simply added to thedrum through the opening in first end 120 and the drum rotated for 30 to60 seconds, within which time complete mixture is effected.

In some applications it may be desirable to effect a change in the drychemical mix composition during the system's operation. As describedbelow, means are provided so that the drum can be removed from theassembly for short periods without interrupting the supply of chemicalto dissolution vessel 26. Consequently, to change the composition of thedry chemical mix, the drum can simply be removed, the new material addedand the drum replaced. Alternatively, an opening can be provided insecond end 122 of the drum through which additional material can beadded while the drum is rotating.

CONVEYOR ASSEMBLY

Referring now to FIGS. 2 and 4-8, the transport system used in thepreferred embodiment of the present invention comprises a conveyorassembly 28. Conveyor assembly 28 includes a first end 162 that extendsinto first end 120 of drum 22 and a second, opposite end 164 whichterminates above dissolution vessel 26. In operation, conveyor 28receives dry chemical from chute members 146 as the drum rotates anddeposits this chemical into dissolution vessel 26.

Conveyor assembly 28 includes a looped belt 166 and first and second endrollers 168, 170. First end roller 168 is rotatably mounted about ashaft 178 fixedly positioned in an end piece 180. End piece 180 isslidably positioned with respect to a conveyor frame member 182 by amanual slot tensioning mechanism 183. Slot tensioning mechanism 183comprises a slot 184 in end piece 180, a threaded shaft 186 and wingnuts 188 (FIG. 6). (In alternative embodiments, a spring-loaded selftensioning mechanism could readily be employed.) Conveyor frame member182 links roller 168 to hub 126 and is affixed to the hub by a nut andbolt 190, 192.

Operation of first conveyor roller 168 can be impaired if thehygroscopic chemical mix is allowed to be deposited onto the lowerportion 196 of belt 166 as the belt travels in drum 22. Such chemical onthe lower portion of the belt can become interposed between the belt andthe roller, fouling the roller and interfering with its operation. Inorder to prevent any chemical from being deposited onto the lowerportion 196 of belt 166, conveyor frame member 182 is desirably formedand positioned to serve as a shroud over the lower portion of the belt,as illustrated in FIG. 7. As a further precaution against chemicalfouling of roller 168, an angled scraper blade 222 is affixed to frame182 and impinges against the upper surface of the lower portion of thebelt so as to scrape dry chemical residue off the belt.

A back reaction stop member 200 (FIG. 7) is positioned adjacent belt 166at its first end to accumulate some of the dry chemical that is pouredonto belt 166 from chute members 146 as the drum rotates. Thisarrangement maintains a reserve of dry chemical 201 on the belt so as toprevent the rate of chemical delivery to the dissolution vessel frombeing modulated by the periodic replenishing action of chute members146. Stop member 200 also allows drum 22 to be removed from the assemblyfor short periods without interrupting the supply of chemical todissolution vessel 26. During such periods, conveyor 28 simply drawschemical from the reserve of chemical piled on the belt adjacent member200 with no detectable change in the rate of chemical delivery to vessel26.

Dry chemical dumped onto belt 166 by chute members 146 passes through agate, or plow assembly 230 (FIG. 7) which defines the profile of thechemical carried by the belt to the dissolution vessel. Gate assembly230 includes a plate 232 into which is cut an inverted U or V-shapednotch 234. Plate 232 prevents the passage of dry chemical near the edgesof the belt, allowing only the material passing through the profile ofthe centered cutout gate to pass. Dissolution vessel 26 is therebyprovided with a continuous ribbon of chemical having a constantcross-sectional area. Chemical that does not conform to the profile ispushed off the belt and falls back into the drum.

Gate assembly 230 includes an adjustable member 236 pivotally mounted bya nut and bolt 238 to occlude portions of the cutout gate. By moving theposition of adjustable member 236, the profile of dry chemical carriedto the dissolution vessel can be varied. Although adjustable member 230is illustrated as being pivotally connected to plate 232 by nut and bolt238, in alternative embodiments this adjustable member can be movedautomatically in response to signals from control system 44 so as tovary the rate at which dry chemical is introduced into the dissolutionvessel in response to detected changes in the dialysate conductivity.

Since gate assembly 230 regulates the rate at which chemical is suppliedto the dissolution vessel 26, the rotational speed of drum 22, andconsequently the rate at which chemical is deposited onto the conveyor,is not important. The only constraint on the rotational speed of drum 22is that it should not be so slow that the chemical reserve accumulatedby stop member 200 is exhausted between periodic replenishment by chutemembers 146 if a steady supply of chemical to the dissolution vessel isdesired.

On the other end of conveyor 28, second end roller 170 is keyed to ashaft 172 that is rotated by a motor 174. The speed of motor 174 isgoverned by control system 44, as discussed below. Frictional engagementbetween second end roller 170 and belt 166 is enhanced by a plurality ofO-rings 176 positioned about a recessed central portion of the secondroller. The pressure applied to O-rings 176 by belt 166 causes theO-rings to deform, the outer ones more than the inner ones, therebyforming a slight crown roller which aids in centering of the belt on theroller.

Second end 164 of conveyor 28 is desirably substantially enclosed byside walls 212, 214 and top and bottom walls 216, 218 so as to minimizemoisture transfer from the humid atmosphere above dissolution vessel 26back into rotating drum 22. Side walls 212, 214 also aid in centeringthe belt on second roller 170. Top wall 216 is spaced above belt 166 topermit the desired powder profile to pass unimpeded to dissolutionvessel 26. Lower wall 218 is adjacent belt 166 and desirably includes aninclined edge portion 220 pressing against the belt as it comes offsecond roller 170 so as to remove any powder residue that may cling tothe belt.

CONTROL SYSTEM

Control system 44 monitors the concentration of the resulting dialysatesolution and controls the addition of materials into the dissolutionvessel to regulate this concentration at a desired level.

In the preferred embodiment, control system 44 controls the speed ofmotor 174, and consequently the speed of conveyor 28, in response to thedetected concentration of the dialysate solution. A schematic blockdiagram of a suitable control system is shown in FIG. 9.

In the illustrated control system, electrode 40a in dissolution vessel26 is grounded and electrode 40b is excited with an alternating currentsignal at two kilohertz provided by an oscillator 304 through a voltagedropping resistor 306. The magnitude of the signal provided byoscillator 304 to circuit node 308 is reduced if circuit node 308 isloaded by a low impedance path through the solution in the dissolutionvessel to ground. Consequently, the signal magnitude at circuit node 308decreases if the concentration of the solution increases.

The signal at circuit node 308 is fed to an inverting peak detector andfilter circuit 310 which rectifies the alternating current signal andproduces an output signal inversely proportional to its short term peakvalue. Peak detector and filter circuit 310 can be a simpleresistor-capacitor time delay circuit driving an analog inverter.Alternatively, the peak detection and filtering functions can be omittedentirely, with only a slight degradation in system performance, and asimple analog inverter stage used instead.

The output from inverting peak detector and filter circuit 310 is fed toan LM324 operational amplifier stage 312 configured as a second analoginverter. The noninverting input of amplifier 312 is provided with abias voltage set by a set point control 314. Set point control 314 is acomponent of a voltage divider circuit that provides a reference signalestablishing the constant value at which the solution concentration isto be maintained. Set point control 314 also allows the concentration ofthe dialysate to be varied, as desired, during therapy. A conventionalfeedback circuit 316 is connected from the output of amplifier 312 backto its input to set the gain of the inverting amplifier to the desiredvalue, here twenty.

The output of amplifier 312 is applied to the noninverting input of asecond LM324 operational amplifier 318. The inverting input of amplifier318 is biased by a combination resistive voltage divider and feedbackcircuit 320.

The output of second operational amplifier 318 is fed to an LM317Tbuffer amplifier 322 which is a unity gain, high current device used todrive motor 174. The greater the output signal from second amplifier318, the greater the output signal from buffer amplifier 322 andconsequently the faster motor 174 will turn.

The current drawn by motor 174 is sampled by a current sensing resistor324. The voltage across this resistor is fed back into an LM324 speedstabilization circuit 326. The output of speed stabilization circuit 326is added to the output of first amplifier 312 at the input of secondamplifier 318. This stabilization signal confirms to amplifier 318 thatthe motor has changed speed in response to changes in dialysateconcentration and thereby tends to moderate extreme excursions in theexcitation of the motor. (In alternative embodiments, speedstabilization circuit 326 can be omitted entirely with only a slightdegradation in performance.)

An exemplary operation of the circuit of FIG. 9 may be as follows. Thesystem is initially operating in equilibrium, with the concentration ofthe dialysate corresponding to the level set by set point control 314.This equilibrium is then disrupted when the operator of the apparatusincreases the rate of water flow into the dissolution vessel in order toincrease the rate at which dialysate is produced.

The increased rate of water flow into the dissolution vessel causes theconcentration and the conductivity of the resulting dialysate todecrease. The reduced dialysate conductivity reduces the resistive loadpresented across the output of oscillator 304 and causes the magnitudeof the signal at circuit node 308 to rise. Because peak detector andfilter 310 is configured as an inverter, the increase in signalmagnitude at circuit node 308 causes a decrease in the signal applied tofirst amplifier 312. Since amplifier 312 is configured to have aninverting gain of twenty, a small signal decrease at its input causes alarge signal increase at its output. This increase in output signal iscoupled to the noninverting input of amplifier 318, causing the outputof this amplifier also to increase. The increased amplitude signal fromthe output of amplifier 318 is buffered by buffer 322 and fed to motor174. The increased voltage applied to motor 174 increases its speed.This increase in speed speeds the conveyor system, thereby speeding therate at which powder is introduced into the dissolution vessel. Theconcentration and conductivity of the resulting dialysate is therebyincreased and brought back to its original, desired level.

It should be noted that if ring 50 is omitted from dissolution vessel26, then some of the dry chemical dropped by the conveyor onto thesurface of the fluid in the vessel may be withdrawn immediately throughoutlet port 36 before it has had an opportunity to dissolve. In suchcase, the concentration of the dialysate measured by detector 40 in thedissolution vessel 26 may not accurately reflect the concentration ofdialysate ultimately provided to the dialysis machine. Accordingly, inthis and other situations, it is desirable to measure the concentrationof the dialysate using a second detector at a location remote from thedissolution vessel, as for example, at some distance past flowcontroller 38. By the time dialysate reaches this point, any undissolvedchemical flushed out of the dissolution vessel with the dialysate willdoubtless have dissolved.

As shown in FIG. 9, such a second detector can readily be integratedinto control system 44. A second conductivity probe 42, similar inarrangement to probe 40 in dissolution vessel 26, is inserted in aremote fluid conduit to measure the conductivity of the solution passingtherethrough. This probe is excited by a circuit 330 that includes anoscillator, a voltage dropping resistor and a peak detector/filteridentical to that used in connection with the conductivity probes in thedissolution vessel. In such a two detector system, the detector in thedissolution vessel responds to short-term variations in dialysatecomposition and the remote detector maintains long-term regulation ofthe dialysate composition.

As suggested in the discussion of control circuit operation, the rate atwhich dialysate solution is produced can be varied by manually varyingthe rate at which water is introduced into the dissolution vessel.System control 44 quickly detects any change in dialysate concentrationresulting from a change in the rate of water introduction andautomatically adjusts the rate of dry chemical introduction accordingly.Thus, it is possible in the illustrated embodiment to vary the rate atwhich dialysate solution is produced simply by increasing or decreasingthe rate at which warm water is introduced into the dissolution vessel.

ALTERNATIVE EMBODIMENTS

In an alternative embodiment, control system 44 regulates the dialysateconcentration by varying the rate of water introduction into vessel 26.In such embodiment, the rate at which dialysate solution is produced canbe varied simply by changing the speed at which conveyor belt 28operates. Changes in the rate at which dry chemical is introduced intothe dissolution vessel causes control system 44 automatically to changethe rate of fluid introduction appropriately. Thus, the rate at whichdialysate solution is produced can again be easily varied, while thedialysate concentration is maintained substantially constant.

It should be noted that if regulation of the dialysate concentration iseffected by regulating the rate of fluid introduction into thedissolution vessel, control system 44 must be modified somewhat. In theillustrated control system, a detected decrease in dialysateconcentration causes the speed of motor 174 to increase so that itprovides chemical to the dissolution vessel at a faster rate, therebyincreasing the dialysate concentration. In contrast, if control overdialysate concentration is effected by controlling the rate of fluidintroduction, a detected decrease in dialysate concentration must causethe motor driving the fluid pump to operate at a slower rate in order toincrease the dialysate concentration. This change in control systemresponse can be implemented by constructing peak detector and filtercircuit 310 to be a non-inverting stage, rather than the inverting stageillustrated.

In some embodiments, the pressure at which the fluid is supplied to theapparatus from fluid source 24 may be sufficient to generate an adequateflow of fluid into the dissolution vessel without pump 30. In suchembodiment, regulation of the fluid flow can be effected by a valveinterposed in fluid inlet conduit 31. This valve can be responsive tocontrol system 44 if dialysate concentration is regulated by changingthe rate of fluid introduction. If the dialysate concentration is toohigh, the output signal from the control system can open this valvefurther, and vice versa.

With certain bicarbonate dialysate solutions, it is desirable tointroduce an acidic fluid into the dialysate solution so as to controlsolution pH and to prevent the sodium bicarbonate from precipitating. Asnoted earlier, such systems can employ an acidic fluid source 34(FIG. 1) and a pump 32 for introducing an acidic fluid, such as aceticacid, into fluid inlet conduit 31. Pump 32 is desirably a variable speedpump so that the rate of acidic fluid introduction into the solution canbe controlled. In one embodiment, pump 32 is driven from the same shaft172 that drives conveyor 28. The ratio of acidic fluid to dry chemicalis thus always maintained at a constant value regardless of the speed atwhich shaft 172 rotates. (In such embodiment, the ratio of acidic fluidto dry powder can be set to a desired value by moving adjustable member236 on powder gate 230 so as to set the rate of dry chemical deliveryfor a given belt speed.)

Although the preferred embodiment of the present invention has beendescribed with reference to detectors 40 and 42 that measure theelectrical conductivity of the resulting solution, detectors 40 and 42could alternatively measure other parameters. For example, the detectorscould measure the pH or the color of the solution by using conventionaltechniques. Regardless of what characteristic is detected, controlsystem 44 can be used to responsively alter the rate at which thecomponents being mixed are introduced so as to maintain the detectedcharacteristic at a desired value.

Similarly, although the preferred embodiment of the present inventionhas been described with reference to a dissolution vessel in which thelevel of fluid is maintained constant by the position of fluid outputport 36, alternative arrangements can readily be devised. For example,solution can be withdrawn through a conduit in the vessel or by a fluidoutput port in the midportion of the vessel. In such cases, the level offluid in the vessel can be regulated by conventional electrical oroptical level sensors. If these level sensors detect that the fluidlevel is above a desired level, the rate of fluid withdrawal through theoutlet port is increased, and vice versa.

In still other embodiments, fluid output port 36 in the side of vessel26 can be omitted and the fluid can be allowed instead to overflow thetop of the dissolution vessel and fall into a surrounding capturevessel. In such a system, the volume of fluid in the dissolution vesselis necessarily constant, regardless of changes in flow through the flowcontroller.

In application, the illustrated embodiment can be used as a dialysatedelivery unit for a single patient dialysis machine or can be used as acentral dialysate delivery unit serving several machines. The inventioncan also be used to retrofit existing dialysis machines, replacing thecomplex liquid dialysate concentrate proportioning systems currentlyused with a simpler, more economical dry chemical based system.

Finally, while the embodiment has been described with repeated referenceto continuous operations, such as the continuous introduction of powderinto the dissolution vessel, it will be recognized that substantiallythe same result is achieved by operating these processes intermittently,at a regular periodic rate. Thus, for example, the motor driving thepowder conveyor can be driven in steps by a stepper motor havingstationary periods, rather than being run continuously.

Having described and illustrated the principles of our invention in apreferred embodiment it should be apparent that the invention can bemodified in arrangement and detail without departing from suchprinciples. We claim all modifications coming within the scope andspirit of the following claims and equivalents thereof.

We claim:
 1. An apparatus for dissolving a dry powder in a liquid comprising: a tapered dissolution vessel having a first, upper inlet means for receiving dry powder to be dissolved and a second, lower inlet means smaller than the first for receiving a flow of incoming liquid into which the dry powder is to be dissolved, the vessel further including outlet means distinct from said first and second inlet means for permitting the withdrawal of dissolved solution from the vessel, the vessel including a principal vertical axis along which the lower inlet means is spaced from the outlet means; whereby powder introduced into the first inlet means that does not dissolve at the surface of the fluid gravitates down towards the second inlet means at which dissolution of the powder is facilitated by virtue of the increased purity and the increased flow per unit area of the incoming liquid.
 2. The apparatus of claim 1 which further comprises:liquid supply means for introducing a flow of incoming liquid into the second inlet means; sensing means for sensing the concentration of the dissolved solution and for producing an output signal corresponding thereto; and regulation means responsive to said output signal for varying the rate at which the liquid is introduced to the second inlet means so as to maintain the concentration of the solution at a substantially constant value.
 3. The apparatus of claim 1 which further comprises:dry powder supply means for providing dry chemical to the first inlet means; sensing means for sensing the concentration of the dissolved solution and for producing an output signal corresponding thereto; and regulation means responsive to said output signal for varying the rate at which dry chemical is provided to the first inlet means so as to maintain the concentration of the solution at a substantially constant value.
 4. The apparatus of claim 3 which further includes reference means coupled to the regulation means for establishing the constant value at which the solution concentration is to be maintained.
 5. The apparatus of claim 1 which further comprises:withdrawal means for withdrawing solution from the output means; and flow regulation means for regulating the rate at which solution can be withdrawn from the output means.
 6. The apparatus of claim 5 which further comprises a second vessel for containing solution withdrawn from the dissolution vessel, said second vessel including the flow regulation means and further comprising bubble nucleation means for causing gas in the withdrawn solution to form bubbles.
 7. The apparatus of claim 1 which further comprises:a drum for containing dry powder and being rotatable about an axis; conveyor means extending into said drum through an end thereof for conveying dry powder from the drum to the dissolution vessel; and baffle means in the drum and responsive to its rotation for directing dry powder onto the conveyor means.
 8. The apparatus of claim 7 which further includes gate means for regulating the cross-section of dry chemical carried by the conveyor means from the drum to the dissolution vessel.
 9. The apparatus of claim 1 in which the outlet means is a weir over which the solution flows to regulate the level of liquid in the dissolution vessel.
 10. The apparatus of claim 9 which further includes means for preventing powder introduced onto the surface of the liquid from flowing out of said vessel through said weir without first sinking into the liquid.
 11. In an apparatus for producing dialysate used in hemodialysis therapy, an apparatus for dissolving dry chemicals into a stream of water to produce a dialysate solution, comprising:a vertically oriented dissolution vessel having sides defining a top orifice and a bottom orifice smaller than the top orifice and situated below the top orifice, where at least a portion of the sides taper from the top orifice to the bottom orifice; first supply means for supplying the water into the dissolution vessel through the bottom orifice; second supply means for supplying the dry chemicals through the top orifice into the water in the dissolution vessel as the water is supplied through the bottom orifice so as to dissolve the chemicals in the water to form the dialysate solution; outlet means for withdrawing the dialysate from the dissolution vessel as the dialysate is formed therein; detector means for detecting a characteristic of the dialysate solution and for producing an output signal corresponding to the characteristic; and means responsive to the detector means output signal for cooperating with at least one of said supply means to regulate the detected characteristic of the dialysate solution at a desired value as the dialysate solution is being formed.
 12. The apparatus of claim 11 in which the detected characteristic is the conductivity of the solution.
 13. The apparatus of claim 11 in which the detected characteristic is the pH of the solution.
 14. The apparatus of claim 11 in which the detected characteristic is the color of the solution.
 15. In a method of dissolving a powder into a liquid to yield a continuous supply of solution, an improvement comprising the steps:providing a tapered dissolution vessel having a bottom orifice into which undissolved powder introduced to the vessel gravitates; and preventing undissolved powder from settling in said vessel by introducing the liquid into the vessel through said orifice.
 16. The improvement of claim 15 which further comprises the steps:providing a dissolution vessel that tapers from a wide mouth position to a narrow neck portion; orienting said vessel with said narrow neck portion beneath said wide mouth portion; introducing the powder into the wide mouth portion; and introducing the liquid into the narrow neck portion. 